At 12:18 AM 12/1/2011, you wrote:
>Hi all,
>
>I have a Hammond 1551R box into which fits a small PCB.
>The recommended shape of the PCB is given in the datasheet:
>(http://www.hammondmfg.com/pdf/1551R.pdf)
>I have received the PCB and it fits perfectly, but the Hammond box only
>came with the 2 screw for the lid and none for the PCB mount holes. The
>sizes specified are 2.5mm for the PCB hole diameter, but the screws are
>given as 32 x 3/16" self tapping.
>So the question is - what does this translate to in metric sizes? I get
>the 3/16" is probably the length, but what does 32 stand for? Is it for
>a 1/32" thread?

I don't see a "32" on that drawing, I see #2 x 3/16"  32 threads per
inch (North American screws use the reciprocal of the pitch) is certainly
plausible- typically this type of screw has a much coarser pitch than the
equivalent diameter machine screw.

In any case, this would be approximately a M1.8 Thread Forming fastener,
also called K18, but you'd have to verify the size to be sure. Pitch at
0.8mm is about right.

Here's one maker, along with some design rules for various plastics:-
http://www.semblex.com/files/PT-Semblex.pdf

Also note the Hammond part number 1551ATS100 for 100 pieces of screw,
about 6 cents each.. if you don't feel like screwing around with substitute=
s.

I would not be particularly gobsmacked if they were actually metric--
K18 4.5 or 5mm.

>I have trawled through pages and pages of all the many different
>"standards" - Whitworth, BAS, UTS, etc but I can't fit 32 into anything
>that seems to fit properly.

Thread forming screws designed for plastics have a completely different
thread form from machine screws. Also different from those designed for
metal. You might be able to use ones designed for metal, but they won't be
ideal.

>The UTS (Hammond are US so I assumed this would be the most likely) is

Hammond is Canadian, headquartered in Guelph, Ontario.,

>specified as X-Y where X is the diameter and Y is the thread turns per
>inch. 1/32 would give a diameter of 0.8mm and 3/16 means nothing in
>terms of turns per inch. Even if you swap them round it doesn't make sense=
..
>Whilst linking to the above datasheet I noticed it's actually different
>from the one I have, and gives #2 x 3/16" instead of the aforementioned
>32 x 3/16". This makes more sense as #2 equates to 2.18mm diameter
>shank, but I'm still unsure as to the thread (not that it matters too
>much as it's self tapping, but I'd like to know anyway)
>I'm going to order some number 2 screws, but can anyone help me to
>confirm exactly what the numbers mean for future reference?

Hope this helps.

--=20
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